The present invention relates to a magnetic recording medium such as a magnetic tape. More particularly, it is concerned with a magnetic recording medium of so-called coating type which consists of a non-magnetic substrate and a magnetic layer formed thereon by the application of a magnetic paint composed mainly of a ferromagnetic powder and binder.
The magnetic recording medium such as a magnetic tape or a magnetic disc, which medium is in general use, is of so-called coating type. It consists of a non-magnetic substrate such as polyester film and a magnetic layer formed thereon by the application of a magnetic paint prepared by intimately mixing a fine ferromagnetic powder, resin binder, organic solvent, and a variety of additives. It is improving in performance to meet the requirement for higher recording densities. For this reason, the conventional ferromagnetic powder is being replaced by a new one which has a smaller particle diameter and a larger specific surface area. Moreover, there is appearing in the industry an ultrafine ferromagnetic powder made of a metal (such as Fe, Ni, and Co) or an alloy thereof which has superior magnetic properties.
The magnetic recording medium of the coating type should meet several requirements. That is, the ferromagnetic powder having an extremely small particle diameter should be uniformly dispersed into a binder resin. The magnetic layer should have improved surface properties so that it exhibits improved electromagnetic conversion characteristics. The magnetic coating film should have improved durability. In other words, a very important problem associated with the magnetic recording medium of the coating type is how to disperse a ferromagnetic powder uniformly into the magnetic layer and how to establish a good bond between the ferromagnetic powder and the binder.
A conventional way to meet these requirements was to add a dispersing agent (such as lecithin) to the magnetic paint or to introduce a polar group into the binder. However, this is not satisfactory for the complete dispersion of a fine ferromagnetic powder having a specific surface area larger than 45 m.sup.2 /g, because the finer the ferromagnetic powder becomes, the greater the cohesive force it has. The addition of a dispersing agent (such as lecithin) is liable to lower the coating film strength because of its weak affinity to the binder and hence poor interfacial reinforcement.
The uniform dispersion of a ferromagnetic powder is also necessary for good runnability because it provides the magnetic layer with a smooth surface which has a substantially low coefficient of friction. A conventional measure to meet this requirement was to add a dispersing agent (such as lecithin) to the magnetic paint, to introduce a polar group into the binder, or to add carbon to the magnetic layer. The addition of carbon, however, does not fully exhibit its effect in the case of a magnetic layer containing a fine ferromagnetic powder uniformly dispersed therein. A large amount of carbon has to be added to lower the coefficient of friction to a certain extent; this deteriorates the electromagnetic conversion characteristics instead.